MG Car Repair: Shooting oil from breather hose, crankcase pressure, vent tube

Question
Hi, Ive got a 78 MGB with a downdraft weber. The breather from the side of the engine tappet cover into the aircleaner and when i drive down the highway around 3500 to 4000 rmp's it spits oil in the carb. I took it off the aircleaner and it spits oil on the inner fender.  the oil level is not over full. Is there another vent or something that im missing that could be blocked or something?

Answer
Hi Scott,
The crankcase vent from the tappet cover has a can inside as part of the cover and inside that can is a metal mesh which is suppose to condense oil vapors as they try to exit the vent tube. Then drain that condensed oil back into the case. The valve cover vent tube has an orifice at the end of the tube to prevent too much air entering the air cleaner and thus drag too much oil vapors into the air filter.

There are several reasons the system breaks down and excess oil escapes the crank case. The most common is that the rings have too much blow-by and the crankcase pressure builds too high. Another possible is that the oil is too thin do to fuel contamination or just too thin of a grade of oil was used. Another reason can be that the oil is getting too hot either because the combustion chamber is over heated or the oil cooler is not operating. Even a too hot thermostat may be used.

Any one or any combination of several above faults can be the cause. Most of my MG dealership experience was in FL and most MGBs could cruise all day in the summer at 3500 to 4000 RPM without having a venting problem.

If it were mine I would start testing each item I listed. Starting with a dry and a wet compression test or if you can find someone with a "Leak-Down" tester to check the rings first. Then confirm correct ignition timing at 4000 RPM and check for a carburetor lean condition at 4K RPM. Check the temp of the thermostat and install a colder thermostat like a 165 deg. And clean out the oil cooler (if it has one) (if not get one) and be sure you don't use any 5w oil. If it has low mileage use 10w 30 oil or if it is a high mileage engine use 20w 40 oil with a can of STP oil treatment.
Howard